1. Field of the Invention
The present invention relates to an electromagnetic continuous casting apparatus for producing a high-purity polycrystalline ingot of semiconductive materials or metallic materials having a high melting temperature and a low electric conductance such as a silicon ingot used for a solar cell substrate.
2. Background of the Related Art
In general, the aforementioned type silicon ingot is used as a starting material for manufacturing silicon wafers, for example, which are used as photovoltaic elements such as solar cells.
Conventionally, in order to manufacture a polycrystalline silicon wafer for solar cells, firstly, silicon is melted inside a graphite crucible or inside a quartz crucible placed inside a graphite crucible. Thereafter, an ingot is produced by slowly cooling the mold (or crucible) from the bottom thereof such that the melt can be directionally solidified. The ingot is sliced into wafers having a thin thickness of below 500 μm. This method has been widely used, but embraces some problems in that the silicon ingot is contaminated by carbon and oxygen introduced from the graphite or quartz crucible, and the quartz crucible is fractured during solidification of silicon and must be replaced with a new one every time to thereby increase the manufacturing cost. On the other hand, the quality and cost of silicon wafers depend on those of the ingot. Therefore, recently, an electromagnetic continuous casting process has been introduced, in which the introduction of impurities can be suppressed to thereby improve the quality of silicon wafer, and loss of the crucible and mold can be alleviated to thereby improve its productivity and consequently reduce the manufacturing cost.
FIG. 1 schematically shows a conventional electromagnetic continuous casting machine. As shown in FIG. 1, the conventional electromagnetic continuous casting machine includes an induction coil 1, and a continuous casting-type cold crucible 2 disposed inwards of the induction coil 1. The cold crucible is made of a conductive material (commonly, oxygen free copper, OFC) and opened at its bottom.
FIG. 2 is a perspective view showing a cross-section of the cold crucible in the conventional electromagnetic continuous casting machine in FIG. 1. Referring to FIGS. 1 and 2, the cold crucible 2 is structured in such a manner that at least part of the crucible along the circumference thereof is divided into plural segments by vertical slits 3. For the purpose of solidification of a melt and protection of the cold crucible 2, a water cooling system is provided, by which a cooling water passes inside the crucible.
Due to these longitudinal slits 4, the electromagnetic field, which is generated by a high-frequency current of the induction coil 1, can be permeated into the inside of the cold crucible 2 and generates an induced current in the melting material. Accordingly, the melting materials, which are continuously charged, are heated and melted by means of the joule heating effect. Simultaneously, an electromagnetic force is always generated towards the inside of the cold crucible 2 such that the contact between the melting materials and the inner wall of the crucible can be alleviated. This phenomenon due to the electromagnetic force is called pinch effect or electromagnetic pressure effect.
In the electromagnetic continuous casting process, since the contact with the crucible is suppressed as described above, contamination of the melt is alleviated and thus the quality of ingot can be improved. At the same time, the mold does not need to be replaced, thereby reducing the installation cost thereof and improving the productivity therefor. In addition, it has an advantage of enabling a continuous casting and providing a good production efficiency, thereby manufacturing an economically efficient ingot.
However, since the above conventional electromagnetic continuous casting process uses a water-cooled cold crucible, it has difficulties in making a material, which has a high melting temperature and a low electric-conductivity, a high-purity ingot. That is, while in case of steel or aluminum materials which do not necessitate high purity, there exists an antecedent process for melting the raw materials before casting and the cold crucible is wholly used for casting process, in case of a high-purity ingot, the melting process and the casting process should be carried out simultaneously and continuously inside the cold crucible. Therefore, a great deal of electric power is required for melting the raw materials.
In particular, since silicon is a semiconductive material having a high melting point and a low electric conductivity, a cooling effect by the emission of radiant heat is high, but, in contrast, the induction heating effect is weak. Thus, it causes difficulties in melting the silicon material continuously and efficiently.
In order to solve the above problems in the art, currently a high capacity power source of around several hundreds kW has been used. Alternatively, Japanese Laid-open Patent Application No. 2001-19594 discloses a method of forming a silicon melt inside a bottom open type crucible, in which an additional plasma arc-heating source is used, together with the induction heating by an induction coil, which is disposed around a crucible. However, this method leads to a high installation cost and also a high production cost disadvantageously.